1. Field of the Invention
This invention relates to an in-vehicle load drive-controlling device for controlling energization of an in-vehicle electric load, and, more particularly, to an in-vehicle load drive-controlling device for protecting a power switching element such as a power MOSFET from an excessive current.
2. Description of the Related Art
A previously known circuit for protecting a power MOSFET for drive-controlling the in-vehicle electric load from an excessive current is disclosed in, e.g., JP-A-2000-193692 which proposes an excessive current detecting circuit and an excessive current detecting/protecting circuit.
FIG. 21 shows a circuit showing the structure of a conventional protecting circuit.
In FIG. 21, a series circuit composed of a reference resistance Rr and a reference MOSFET QB, which constitutes a reference circuit, is connected in parallel with a series circuit composed of a load L and power MOSFET QA. The reference resistance Rr and the reference MOSFET are equivalent to the load L and the power MOSFET QB, respectively. On the basis of a difference between the drain-source voltage of the reference MOSFET QB through which a reference current flows and that of the power MOSFET QA in which the current is varied by an excessive current, the excessive current flowing through the power MOSFET is detected.
Such an excessive current detecting circuit is implemented as an integrated circuit so that the reference MOSFET QB and the power MOSFET QA are formed on the same chip by the same process and both of them are composed of a plurality of transistors.
In a conventional excessive current detecting circuit, because the power MOSFET QA, which is a switching element, and a reference power MOSFET QB, which is also a switching element, are integrated to form an IC excessive current detecting circuit, the structure of each element is complicated and the control circuit therefore is also complicated.
In the circuit diagram of FIG. 21, if there is a difference between the ground potential of the load L and that of the reference resistor Rr, an error in the detected current is generated. This makes it impossible to protect the switching element from an excessive current.
The gate control circuit for the power MOSFETs QA and QB, which includes MOSFETs Q1 and Q2, a comparator CP, etc., and a loadine are coupled with each other by a terminal T3. Therefore, the control circuit may malfunction or may be broken due-to as a result of the noise from the load line.
This invention has been accomplished in order to overcome such an inconvenience.
An object of the present invention is to provide an in-vehicle electric load drive-controlling device which can be simplified in circuit configuration and improved in safety and reliability from the viewpoint of protection from excessive current and exclusion of influence of external noise.
In order to attain the above object as seen from FIG. 1A, in accordance with the present invention, there is provided an in-vehicle load drive-control circuit comprising:
a power MOSFET between connected in series between a load L and a power source B, the power MOSFET on/off controlling the power supply to the load, the power MOSFET incorporating a thermoelectric element D across which the voltage drops as a result of heat liberation when the power MOSFET is energized; and
a control means COT for ON/OFF controlling a gate driving signal to the power MOSFET on the basis of a voltage drop,
wherein after the voltage has been stabilized, the gate driving signal is made constant.
In this configuration, the temperature change of the thermoelectric element incorporated in the power MOSFET is detected in terms of a voltage change to detect heat liberation as a result of a current flowing through the MOSFET, and a gate driving signal to the power MOSFET is ON/OFF controlled and is made constant after the voltage of the thermoelectric element has been stabilized. This configuration inhibits the breakage of the MOSFET due to excess current in a simple structure.
Preferably, the control means comprises a rush current detecting unit DT11 for detecting a rush current to the load on the basis of a time changing rate of the voltage drop to produce an interrupting signal for the gate driving signal to the power MOSFET.
In this configuration, when the abrupt voltage drop across the thermoelectric element is detected as a result of the rush current flowing through the load which is ten times as large as the rated current when the power supply to the load is initially turned on, the gate driving signal is interrupted, and after the voltage across the thermoelectric element rises to the stationary level, the gate driving signal is produced. Such a configuration inhibits the breakage of the MOSFET due to the excess rush current when the power MOSFET is initially turned on.
Preferably, the control means comprises an abnormal current detecting unit DT12 for deciding an excess current due to poor wiring when the number of times of detecting the rush current by the rush current detecting unit is more than a prescribed number to produce an interrupting signal for the gate driving signal to the power MOSFET.
In this configuration, if the abrupt voltage drop across the thermoelectric element is detected whenever the gate driving signal is supplied to the power MOSFET, under the decision that this is attributed to short-circuiting of the wiring, the gate driving signal to the power MOSFET is interrupted. Such a simple configuration inhibits the breakage of the power MOSFET due to the excess current when the wiring is short-circuited
Preferably, the rush current detecting unit DT11 produces the gate driving signal at intervals while a load current is suppressed to less than a prescribed current after the voltage across the thermoelectric element has dropped to a prescribed voltage to repeat an ON/OFF operation of the power MOSFET so that the voltage is increased by a certain degree by heat dissipation of the power MOSFET.
In this configuration, heat concentration to the thermoelectric element is diffused to increase the voltage by a certain degree. Thus, the restoration of the operation of the power MOSFET can be performed instantaneously and smoothly.
Preferably, the rush current detecting unit produces the gate driving signal at intervals while a load current is suppressed to less than a prescribed current after the voltage across the thermoelectric element has dropped to a prescribed voltage to repeat an ON/OFF operation of the power MOSFET is repeated and ON/OFF operation of energization of a load so that heat liberation of the load is stabilized to make resistance constant.
In this configuration, the rush current due to the initial low resistance of the load can be inhibited and the restoration of the operation of the power MOSFET can be performed instantaneously and smoothly.
Preferably, the control unit includes an overheat detecting unit DT13 for detecting an overheat abnormality of the power MOSFET when the voltage drops to produce an interrupting signal for the gate interrupting signal.
In this configuration, the excess current supplied to the power MOSFET can be detected in a simple structure, and breakage of the power MOSFET due to the excess current can be inhibited.
Preferably, the overheat detecting unit DT13 decides restoration of the overheat abnormality of the power MOSFET when the voltage rises, thereby producing the gate driving signal.
In this configuration, the restoration of the power MOSFET can be performed smoothly.
Preferably, the thermoelectric element D is a diode whose forward voltage increases with an increase in an ambient temperature. In this configuration, since the diode can be manufactured on the same pellet in the same manufacturing step as the MOSFET, the overheat of the MOSFET due to the excess current can be effectively detected.
Preferably, as seen from FIG. 1B, the in-vehicle load drive controlling device further includes a plurality of power MOSFETs Q1 to Q4 for driving a plurality of loads LM1 to LM4, respectively, and the control unit supplies gate driving signals at slight intervals over time to the gates of these MOSFETs.
In this configuration, the currents on the power source lines are averaged so that the burden for the battery and the loss of the current lines is reduced. The radiation noise due to a current change can be also reduced.
Preferably, the control unit supplies the gate-driving signal based on a PWM signal to the power MOSFET.
In this configuration, the power source voltage can be adjusted to the voltage level of the load by controlling the duty ratio of the PWM signal. After the rush current has been detected, the gate driving signal is intermittently supplied to the power MOSFET on the basis of the PWM signal without performing a timer operation.
The above and other objects and features of the invention will be more apparent from the following description taken in conjunction with the accompanying drawings.